Multiple, geographically dispersed antennas connected to a central baseband processing unit are used as a cost-efficient way of building networks. With the base band processing located in a single node, coordinated multi-point (CoMP) transmission/reception can be deployed. In the downlink, transmissions from multiple transmission points are coordinated. Depending on to what extent the terminals are aware of transmissions originating from multiple points, three different alternatives can be envisioned. In the first alternative A, the terminals are not aware of the transmission originating from multiple, geographically separated points. The same receiver processing and measurement reporting as for single-point transmission is therefore used. Hence, in principle, the introduction of multi-point transmission can be made in a backward compatible way, benefiting preexisting LTE (Long Term Evolution) terminals. The network can, e.g., based on existing path loss measurements, determine from which transmission points to transmit to a specific terminal. As the terminals are not aware of the presence of multipoint transmission, UE (User Equipment)-specific reference signals are used for channel estimation. In this setting, CoMP provides diversity gains similar to those found in single-frequency broadcast networks and results in improved power amplifier utilization in the network, especially in a lightly loaded network where otherwise some power amplifiers would be idle.
In the second alternative, the terminals provide channel-status feedback to the network for all downlink channels visible to a particular terminal while the receiver processing remains the same as for single-point transmission. At the network side, as all processing is located in a single node, fast dynamic coordination of the transmission activity at the different transmission points is possible. For example, the signal transmitted to a particular terminal can be spatially pre-filtered to reduce inter-user interference. This type of CoMP transmission can in principle provide similar benefits as the first alternative described above, but in addition to improving the strength of the desired signal, the second alternative also allows for coordinating the inter-user interference to further improve the SNR (Signal to Noise Ratio). Since the terminal is not aware of the exact processing in the network, UE-specific reference signals are needed.
In the third alternative, the channel-status reporting is the same as the second alternative. However, unlike the second alternative, the terminals are provided with knowledge about the exact coordinated transmission, e.g., from which points, with what transmission weights, etc. This information can be used for received signal processing at the terminal side, but comes at a cost of increased downlink overhead.
Relaying for LTE-Advanced systems improves the coverage of high data rates, group mobility, temporary network deployment, cell-edge throughput and/or to provide coverage in new areas. Type-I relay nodes are part of LTE-Advanced, and a type-I relay node is an in-band relaying node connecting to the eNB (enhanced NodeB) using the LTE spectrum. The relay is connected to the eNB over a backhaul link, and assists the eNB in communicating with a UE terminal over an access link between the relay and the terminal. For a type-I relay, the transmission on the backhaul link (i.e., eNB-to-relay) and the transmission on the access link (i.e., relay-to-UE) are independent. That is, the relay receives data from the eNB over the backhaul link and then forwards the data to the corresponding UE over the access link. As such, the UE views the relay as an eNB.
CoMP can be used for the transmission on the backhaul link since the relay assisting the eNB uses LTE techniques and the LTE spectrum. Similar to normal CoMP transmission between eNBs and UEs, CoMP transmission on the backhaul link also requires CSI about the backhaul link to be available at the eNB., e.g., to update the backhaul link transmission scheme and/or modify the precoding scheme to improve the backhaul link data rate to the relay.
In some cases, a UE estimates the channel responses on some frequency sub-carriers, quantizes the channel responses to digital bits, and feeds back the quantized CSI to the eNB. In response, the eNB reconstructs the channel response for the entire bandwidth. In other cases, each UE may report CSI on only a part of the entire bandwidth. By doing so, the total feedback bits can be reduced. In yet other cases, the total available feedback bits are allocated to different links based on their long term statistics. With this scheme, the feedback mechanism can be used more efficiently. CSI feedback schemes that comprise long-term and short-term feedback mechanisms have been proposed for the access link between the eNB and UE. For these solutions, second-order statistics (e.g., power delay profile) of the channel are considered as the long-term, slow-varying part. Only the second-order statistic is considered because the channel between the eNB and UE typically has little or no line-of-sight components, and thus feedback of first-order channel statistics is not an efficient use of network resources.
Each of these conventional CSI feedback schemes are designed mainly for CoMP transmission between eNBs and UEs. Directly applying these CSI feedback schemes to the backhaul link between an eNB and a relay assisting the eNB is not efficient because the channel characteristics of the backhaul link may differ extensively from those of the eNB-to-UE link. For example, relays are typically fixed once deployed. Hence the channel responses between eNBs and relays change rather slowly, at least for the slow-changing part. The slow varying property provides the possibility to feed back CSI less frequently, at least for the slow-changing part. In addition, relays are typically deployed with line of sight (LOS) to the corresponding eNB. Such a LOS channel has smaller delay spread compared with an eNB-to-UE channel, which in turn leads to larger coherent bandwidth compared to the eNB-to-UE channel. Less CSI feedback is therefore needed for a given bandwidth. Furthermore, relays can only feed back CSI in uplink backhaul subframes. Due to the time division mechanism between the backhaul link (relay-to-eNB) and the access link (relay-to-UE), there are fewer uplink backhaul subframes available than eNB-to-UE uplink subframes. Hence relays have less resource to report CSI compared with a UE, and thus desirable for relays to feed back CSI more efficiently. Conventional CSI feedback schemes are designed based on the eNB-to-UE link, and properties of the backhaul link (relay-to-eNB) are not considered. Such schemes are not efficient for the backhaul link if applied directly.